Method for transmitting data from RLC layer in radio communication system

ABSTRACT

Disclosed is a method for transmitting data from an RLC layer in a radio communication system. The method includes the steps of storing RLC SDUs transferred from an upper layer in a transmission buffer, transmitting RLC PDUs on which the RLC SDUs stored in the transmission buffer are loaded to a receiving side in order, discarding the RLC SDUs stored in the transmission buffer, checking whether a transmission mode indicator is configured, and transmitting discard information about the discarded RLC SDUs to the receiving side entirely or in part in accordance with whether the transmission MRW mode indicator is configured.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of U.S.application Ser. No. 12/425,160, filed Apr. 16, 2009, now U.S. Pat. No.8,004,986, issued on Aug. 23, 2011, U.S. application Ser. No.11/314,240, filed Dec. 22, 2005, now U.S. Pat. No. 7,539,137, issued onMay 26, 2009, and U.S. application Ser. No. 09/971,932, filed Oct. 9,2001, now U.S. Pat. No. 7,054,270, issued on May 30, 2006, and claimsthe benefit of Korean Patent Application No. 2000-59017, filed on Oct.7, 2000 and Korean Patent Application No. 2001-27269, filed May 18,2001, each of which are hereby incorporated by reference as if fully setforth herein.

BACKGROUND OF THE INVENTION

1. Field

The present invention relates to a method for transmitting data from anRLC layer in a radio communication system, and more particularly, to amethod for carrying out a SDU discard function in a radio communicationsystem having an RLC layer.

2. Background

FIG. 1 illustrates an architectural diagram of a general radiocommunication system, having an architecture of a radio interfaceprotocol according to a radio access network standard of an EuropeanIMT-2000 UMTS (universal mobile telecommunication system).

Referring to FIG. 1, a radio link control (RLC) layer in a radiocommunication system as a second layer of 3GPP is a protocol layercontrolling a data link. Moreover, the RLC layer corresponds to a secondlayer of an OSI 7-layered model. All entities of the RLC layer areembodied by a radio resource control (RRC) layer as an upper layer. Theradio interface protocol between a user equipment and a UTRAN (UMTSterrestrial radio access network) includes a physical layer, a data linklayer, and a network layer horizontally. Vertically, the protocolarchitecture is divided into a control plane for transferring a controlsignal (signaling), and a user plane, for transmitting data information.

Specifically, as shown in FIG. 1 a RRC layer exists on the control planeas a third layer, a RLC layer and a medium access control (MAC) layerexist as second layers, and a physical layer exists as a first layer.Moreover, in the user plane, RLC and MAC layers exist on the secondlayer, and the physical layer exists on the first layer.

The physical layer provides the upper layer with an information transferservice using various radio transmission techniques and is connected tothe MAC layer on the upper layer via transport channels. Data istransported between the MAC and physical layers through the transportchannels. The transport channels are divided into a dedicated transportchannel, used exclusively for the user equipment, and a common transportchannel, used for the user equipments in common.

The MAC layer provides a re-allotment service of a radio resource andMAC parameters. Such a service demands the re-allotment of the radioresource or the change of the MAC parameters. The MAC layer is connectedto the RLC layer through a logical channel and provides various logicalchannels in accordance with the species of the transmission information.Generally, the control channel is used when information of the controlplane is transmitted, while a traffic channel is used for transmittinginfo nation of the user plane.

The RLC layer provides the establishment and release of a radio link andperforms segmentation and concatenation relating to a RLC service dataunit (SDU) coming down from an upper layer of the user plane. The sizeof the RLC SDU is adjusted to fit a processing capacity in the RLClayer. Header information is then added to the SDU to form a protocoldata unit (PDU). The RLC SDU is transferred to the MAC layer. In thiscase, an RLC buffer for storing RLC SDUs or RLC PDUs coming down fromthe upper layer exists in the RLC layer.

The RRC layer provides an information broadcast service for broadcastinginformation to all user equipments located at random areas. Moreover,the RRC layer takes charge of a control plane signal processing for acontrol signal exchange in the third layer to establish, maintain, andrelease a radio resource between the user equipment and UTRAN.Specifically, the RRC layer also has functions of establishing,maintaining, and releasing a radio access bearer channel as well asallotment, rearrangement, and release of a radio resource necessary fora radio resource access. In this case, the radio access bearer means aservice provided by the second layer for data transfer between the userequipment and UTRAN. Namely, ‘establishing one radio access bearer’means that characteristics of a protocol layer and a channel requiredfor providing a specific service are stipulated and that the respectivespecific parameters and operational methods are established.

The RLC layer will next be described in greater detail. In accordancewith the functions carried out by the RLC layer, there exist threemodes. These are the transparent mode, the unacknowledged mode, and theacknowledged mode.

When the RLC operates in the transparent mode, no header information isadded to the RLC SDU coming down from the upper layer. Generally,segmentation and reassembly are not performed in the transparent mode.Yet, if necessary, when the radio access bearer is established(configured), it is determined whether the segmentation and reassemblyfunctions are used or not.

When the RLC operates in the unacknowledged mode, retransmission is notbacked up even if the transmission of RLC PDU fails. Therefore, areceiving side does not demand the retransmission when data is lost orwhen problems occur during transmission. Instead, the receiving sidediscards the related data. Services using the unacknowledged modeinclude a cell broadcast service, a voice service using an IP network(voice over IP service), and the like.

When the RLC operates in the acknowledged mode, the retransmission isbacked up when the transmission of a packet fails. Thus, the RLC layerof a transmitting side receives status information from the receivingside used to determine the success of the transmission. The transmittingside RLC layer then retransmits the RLC PDUs demanded forretransmission. In the acknowledged mode, the RLC SDU received by theRLC layer from the upper layer is, if necessary, divided intopre-defined sizes by segmentation/concatenation. The RLC SDU thenbecomes RLC PDUs, to which header information including sequence numbersare added. The RLC PDUs are stored in a RLC buffer according to thesequence numbers.

The stored RLC PDUs amounting to the exact number demanded by the MAClayer are transferred to the MAC layer, where the transmission iscarried out in accordance with the order of the sequence numbers. Fromthe RLC layer of the transmitting side, a firstly-transmitted RLC PDU istransmitted according to the order of the sequence numbers. Therefore,the RLC layer of the receiving side checks the received sequence numbersso as to demand retransmission of transmission-failed data from thetransmitting side RLC layer.

When the RLC SDUs (service data units) having come down to the RLC layerin the transmitting side radio communication system fail to betransmitted to the receiving side radio communication system, thetransmitting side needs to discard the SDUs in order to prevent atransmission buffer from becoming overloaded.

A function of discarding the SDUs (service data units) is used by theRLC layer, which is embodied in the RRC layer as well.

There are two methods for carrying out the SDU discard function. Thefirst method is a time-based SDU discard method. The time-based SDUdiscard method uses a timer to carry out the discarding of SDUs. Thesecond SDU discard method depends on limiting the number oftransmissions. Thus the first method measures the time during which aSDU stays, while the second method measures how many times a PDU isretransmitted. In this case, the target to be discarded is SDU.

Regarding the time-based SDU discard method, when RLC SDUs come down tothe RLC layer from the upper layer of the transmitting side, adiscard-timer is driven. The discard-timer counts the time during whicheach RLC SDU stays in the RLC layer.

Subsequently, if the corresponding RLC SDU fails to be transmitted tothe receiving side before the discard-timer expires, or if an ACK(acknowledgment) signal fails to be received from the receiving sideeven if the corresponding RLC SDU is transmitted to the receiving side,then the receiving side radio communication system discards all of theRLC PDUs including the corresponding SDU.

On the other hand, if an RLC SDU, which is not to be discarded, as wellas the RLC SDU to be discarded are both included in the same RLC PDU,the transmitting side would not discard the corresponding RLC PDU.

The second SDU discard method functions by limiting the number oftransmissions. In the second method, the RLC layer of the transmittingside counts the number of transmissions of the respective RLC PDUstoward the receiving side. Specifically, a counter VT-DAT, which countsthe transmission number of the respective RLC PDUs, is initiated tooperate. The counter VT-DAT increases the count value by 1 every timeone RLC PDU is transmitted to the receiving side.

The transmission-possible number of a specific RLC PDU is defined as amaximum variable MaxDAT. If the count value of the counter VT-DATbecomes equal to or higher than the maximum variable MaxDAT, SDUsincluded in the corresponding RLC PDU and all the RLC SDUs having beentransmitted before the corresponding RLC PDU are discarded.

FIG. 2A illustrates a related art method of transmitting discardinformation of RLC SDUs discarded using the time-based RLC SDU discardmethod to the receiving side.

Referring to FIG. 2A, the RLC layer receives RLC SDU0, RLC SDU1, RLCSDU2, and RLC SDU3 from the upper layer, and transforms them into RLCPDU forms. The RLC PDU forms are successively transmitted to thereceiving side. In this case, the RLC SDUs are data units stipulated bythe upper layer.

The respective SDUs are loaded on at least one RLC PDU so as to betransmitted to the receiving side. SDU0, SDU1, SDU2, and SDU3 includePDU0/PDU1, PDU1, PDU1/PDU2/PDU3, and PDU3 to PDU7, respectively. Thesequence numbers, SN 0 to 7, are given to the PDUs PDU0 to PDU7 inorder, respectively. Thus, the sequence-numbered PDU is transmittedtoward the receiving side.

After the previously-set time during which SDU0 to SDU3 may stay in thetransmitting side expires, PDU0 and PDU1 are transmitted to thereceiving side. PDU2 and PDU3, however, are lost during thetransmission. Meanwhile, PDU4 to PDU 7 are not yet transmitted to thereceiving side from the transmitting side.

As mentioned above, once the previously-set time for SDU0 to SDU3expires, the transmitting side discards PDU0 to PDU7 corresponding toSDU0 to SDU3 in the internal transmission buffer so as to transfer thecorresponding discard information, which is loaded on MRW SUFI (movereceiving window super field), to the receiving side.

FIG. 2B illustrates formats and parameters of the MRW SUFI which aretransmitted in the scenario illustrated in FIG. 2A.

Referring to FIG. 2B, a parameter LENGTH is constructed with 4 bits andindicates the number of discarded SDUs. Other parameters SN_MRW1 toSN_MRW4 indicate sequence numbers of the PDUs respectively. Each of theparameters SN_MRWi (i=1, 2, 3, and 4) represents the sequence number ofthe corresponding PDU including the end of each of the discarded SDUs.And, each of the parameters SN_MRWi (i=1, 2, 3, and 4) is constructedwith 12 bits.

The 12 bit SN_MRWi indicates a sequence number SN of the first PDU in aPDU or PDUs having the data belonging to the SDU next to the ithdiscarded SDU. That is, considering every SDU unit, the SN_MRWi (i is apositive integer) is the information commanding that the first PDUhaving the data of the SDU is to be transmitted right after thediscarded SDU.

As shown in FIG. 2B, the parameter LENGTH of the MRW SUFI indicating thenumber of the discarded SDUs is represented by “0100”. Meanwhile,SN_MRW1 is represented by “1,” which is a sequence number of PDU1, sincean end of SDU0 belongs to PDU1. The SN_MRW2 is represented by “1,” whichis the sequence number of PDU1 since an end of SDU1 belongs to PDU1. TheSN_MRW3 is represented by “3,” which is a sequence number of PDU3 sincean end of SDU2 belongs to PDU3. Finally, the SN_MRW4 is represented by“7” which is a sequence number of PDU7 since an end of SDU3 belongs toPDU7.

A 4 bit NLENGTH field indicates the data, which defines which SDUs areto be discarded, in the PDU having a sequence number of SN_MRWLENGTH.This indicates which SDUs are discarded when a plurality of SDUs areunable to enter one PDU. For instance, when a value of NLENGTH is set to“0”, the PDU having the sequence number of SN_MRWLENGTH shows that thereis no data corresponding to the SDU to be discarded. When a value ofNLENGTH is set to “2”, the data corresponding the second SDU from thefront is discarded and the data in the PDU having the sequence number ofSN_MRWLENGTH is transmitted/received. In FIG. 2A, NLENGTH is representedby “0001”.

The receiving side, which receives the MRW SUFI shown in FIG. 2B asdiscard information from the transmitting side, discards the PDUs PDU0to PDU7 corresponding to SDU0 to SDU3 from a receiver buffer inside andmoves the receiving window. Subsequently, the receiving side transfersan acknowledged signal to the radio communication system correspondingto the transmitting side. The acknowledged signal commands that MRW_ACKdata from the PDU having a sequence number SN of “8” be transmitted tothe transmitting side.

The transmitting side, having received the signal from the receivingside, confirms that PDU0 to PDU7 corresponding to the SDU to bediscarded have been successfully discarded by the receiving side aswell. The transmitting side then starts to transmit the PDUcorresponding to the sequence number SN of “8” and so on to thereceiving side.

The related art method has many problems and disadvantages. For example,when the timer expires or the previously-set number by the counter iscompleted, and all of the SDUs related to the transmission are to bediscarded, all of the information concerning the discarded SDUs istransmitted to the receiving side. As mentioned above, the sequencenumber of the corresponding PDU, which indicates the end point of eachof the discarded SDUs, comprises 12 bits. The transmission efficiency ofthe transmitting side is thus greatly reduced if the discarded SDUs areexcessive.

Specifically, the transmission of the discard information is designed tomake a next receiving window coincide with a next transmission window.Yet, all of the information about the entire discarded SDUs isunnecessarily transmitted to the receiving side. It is enough to informthe receiving side only about the start and end points of a series ofthe discarded SDUs, of which start and end points are known and whichare not transmitted yet, remaining in the internal transmission buffer.Because the excess information is transmitted, a radio resource iswasted.

Moreover, if it is assumed that PDU3 in FIG. 2A has not beentransmitted, when the time for which SDU0 to SDU3 are to stay in thetransmission buffer expires according to the related art timer-baseddiscard method, the RLC layer of the transmitting side discards SDU0 toSDU3 and transmits such information to the RLC layer. Yet, theinformation about SDU3, for which transmission has never been attempted,has a zero probability of being received, but is still transmitted tothe receiving side. Consequently, another radio resource is wasted.

The above references are incorporated by reference herein whereappropriate for appropriate teachings of additional or alternativedetails, features and/or technical background.

SUMMARY OF THE DISCLOSURE

An object of the invention is to solve at least the above problemsand/or disadvantages and to provide at least the advantages describedhereinafter.

An object of the present invention is to provide a system and method oftransmitting data from an RLC layer in a radio communication system thatsubstantially obviates one or more problems due to limitations anddisadvantages of the related art.

Another object of the present invention is to provide a method fortransmitting data from an RLC layer in a radio communication system thatenables more efficient use of radio resources.

Another object of the present invention is to provide a method fortransmitting data from an RLC layer in a radio communication system thatimproves a transmission efficiency of the radio communication system.

Another object of the present invention is to provide a method oftransmitting data from a Radio Link Control (RLC) layer in a radiocommunication system, including storing RLC Service Data Units (SDUs)transferred from an upper layer in a transmission buffer, sequentiallytransmitting RLC Protocol Data Units (PDUs) to a receiving side, thePDUs being formed from the RLC SDUs stored in the transmission buffer,discarding the RLC SDUs stored in the transmission buffer, determiningwhether a transmission mode indicator is configured, and transmittingone of complete discard information and partial discard informationrelating to the discarded RLC SDUs to the receiving side in accordancewith whether the transmission mode indicator is configured.

Another object of the present invention is to provide a method oftransmitting data of a radio link control layer in a radio communicationsystem, including storing upper layer data units transferred from anupper layer in a transmission buffer, forming control layer data unitsof a radio link control layer on which the upper layer data units areloaded, sequentially transmitting the control layer data units to areceiving side, checking transmission failure of the control layer dataunits, one of sending discard information regarding all discarded upperlayer data units and sending discard information regarding a part of thediscarded upper layer data units in accordance with a state of anindicator.

Another object of the present invention is to provide a method oftransmitting data from a Radio Link Control (RLC) layer of acommunication system, including sequentially transmitting RLC ProtocolData Units (PDUs) corresponding to RLC Service Data Units (SDUs), eachPDU having sequence numbers, to a receiving side, discardingcorresponding ones of the RLC SDUs when a previously set reference valueis exceeded, determining whether the discarded RLC SDUs have beentransmitted to the receiving side, renumbering sequence numbers of PDUsassociated with RLC SDUs to be transmitted next so as to be initiatedfrom a sequence number of the first PDU having not been transmitted,determining whether a transmission MRW mode indicator has beenconfigured by an upper layer, transmitting discard information about allof the discarded RLC SDUs to the receiving side when the transmissionMRW mode indicator is configured, and transmitting the discardinformation about a portion of the discarded RLC SDUs to the receivingside when the transmission MRW mode indicator has not been configured.

Another object of the present invention is to provide a message forsending discard information of data of a radio link control layer in aradio communication system, including a first field to indicate a themessage type, a second field to indicate a data unit of a radio linkcontrol layer corresponding to an end of a last discarded data unit ofan upper layer of the radio link control layer, and a third field toindicate a status of the data unit of the radio link control layer.

Another object of the present invention is to provide a user equipment(UE) for a radio communication system, including means for sequentiallytransmitting RLC Protocol Data Units (PDUs) corresponding to RLC ServiceData Units (SDUs), each PDU having sequence numbers, to a receiving sideUE, means for discarding corresponding ones of the RLC SDUs when apreviously set reference value is exceeded, means for determiningwhether the discarded RLC SDUs have been transmitted to the receivingside UE, means for renumbering sequence numbers of PDUs associated withRLC SDUs to be transmitted next, such that they are initiated from asequence number of the first PDU having not yet been transmitted, meansfor transmitting discard information regarding all of the discarded RLCSDUs to the receiving side UE when a transmission MRW mode indicator hasbeen configured, and means for transmitting the discard informationregarding only a portion of the discarded RLC SDUs to the receiving sideUE when the transmission MRW mode indicator has not been configured.

To achieve at least these objects in whole or in parts, there isprovided a method of transmitting data from an RLC layer in a radiocommunication system including storing RLC SDUs transferred from anupper layer in a transmission buffer, transmitting RLC PDUs on which theRLC SDUs stored in the transmission buffer are loaded to a receivingside in order, discarding the RLC SDUs stored in the transmissionbuffer, checking whether a transmission mode indicator is configured,and transmitting discard information about the discarded RLC SDUs to thereceiving side entirely or in part in accordance with whether thetransmission MRW mode indicator is configured.

Preferably, the location information of each of the discarded RLC SDUscomprises a sequence number of each corresponding RLC PDU to which anend of each of the discarded RLC SDUs as well as a sequence number ofRLC PDU from which SDU to be transmitted after each of the discarded RLCSDUs starts.

Preferably, the location information of the last discarded RLC SDU is asequence number of the corresponding RLC PDU to which an end of the lastdiscarded RLC SDU belongs as well as a sequence number of RLC PDU fromwhich RLC SDU to be transmitted after the last discarded RLC SDU begins.

To further achieve at least the above objects in whole or in parts,there is provided a method of transmitting data of a radio link controllayer in a radio communication system, including storing data unitstransferred from an upper layer in a transmission buffer, making dataunits of a radio link control layer on which the data units of an upperlayer are loaded, transmitting data units of a radio link control layerto a receiving side in order, checking a failure of the transmission ofthe data units, sending discard information about all of the discardeddata units of an upper layer or sending discard information about partof the discarded data units of an upper layer according to an indicator.

Preferably, the discard information is sent to the receiving sidethrough a message for moving a receiving window of the receiving side.The message preferably includes a parameter indicating a data unit of aradio link control layer which indicates the end of last discarded dataunit of an upper layer, and a parameter indicating the status of thedata unit of a radio link control layer.

Preferably, the parameter indicating a data unit of a radio link controllayer is the sequence number of the data unit of a radio link controllayer to which an end of the last discarded data unit of an upper layerbelongs or in which the starting point of a data unit of an upper layerto be transmitted after the discard.

Preferably, the parameter indicating the status of the data unit of aradio link control layer is the number of the data units of an upperlayer to be neglected by the receiving side in the data unit of a radiolink control layer. Moreover, the message preferably further includes aparameter indicating that the data unit of an upper layer to bediscarded in the receiver extends above the configured transmissionwindow in the transmitter. Furthermore, the data unit of the radio linkcontrol layer is preferably a RLC PDU and the data unit of an upperlayer is preferably a RLC SDU.

When discard information about part of the discarded SDUs is sent thelast SDU to be discarded at the receiving side is preferably sent as apart of the discarded SDUs. The discard information includes at leastthe information required for moving a receiving window of the receivingside.

To further achieve at least the above objects in whole or in parts,there is provided a method including sequentially transmitting RLC PDUscorresponding to RLC SDUs and having sequence numbers to a receivingside, discarding corresponding ones of the RLC SDUs when a referencevalue set previously to discard each of the RLC SDUs is exceeded,checking whether the discarded RLC SDUs are transmitted to the receivingside, renumbering sequence numbers of PDUs belonging to RLC SDUs to betransmitted next so as to be initiated from a sequence number of thefirst PDU having not been transmitted, checking whether a transmissionmode indicator is configured by an upper layer, transmitting discardinformation about the discarded RLC SDUs to the receiving side entirelywhen the transmission MRW mode indicator is configured, and transmittingthe discard information about the discarded RLC SDUs to the receivingside in part when the transmission MRW mode indicator fails to beconfigured.

To further achieve at least the above objects in whole or in parts,there is provided a message for sending discard information of data of aradio link control layer in a radio communication system, including afield indicating the message type, and a field indicating a data unit ofa radio link control layer which indicates the end of last discardeddata unit of an upper layer of said radio link control layer, and afield indicating the status of said data unit of a radio link controllayer.

The message preferably further includes a field indicating that the dataunit of an upper layer to be discarded in the receiver extends above theconfigured transmission window in the transmitter.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 is a drawing that illustrates an architectural diagram of arelated art radio communication system;

FIG. 2A is a drawing that illustrates a method of transmitting discardinformation of RLC SDUs discarded by the related art time-based RLC SDUdiscard method to the receiving side;

FIG. 2B is a drawing that illustrates formats and parameters of the MRWSUFI which are transmitted according to FIG. 2A;

FIG. 3 is a drawing that illustrates a table of MRW SUFI according to apreferred embodiment of the present invention;

FIG. 4 is a drawing that illustrates a table of MRW SUFI according to asecond embodiment of the present invention;

FIG. 5 is a drawing that illustrates carrying out the discard functionusing the MRW SUFI shown in FIG. 3;

FIG. 6 is a drawing that illustrates carrying out the discard functionusing the MRW SUFI shown in FIG. 4;

FIG. 7 is a flowchart of a method for operating discard functions ofradio communication systems corresponding to transmitting and receivingsides respectively, according to the preferred embodiment;

FIG. 8 is a drawing that illustrates carrying out a discard function forservice data units according to a stay time limit in a transmissionbuffer in a radio communication system according to the preferredembodiment of the present invention; and

FIG. 9 is a flowchart of a method for carrying out a discard functionfor service data units (SDUs) according to a stay time limit in atransmission buffer in a radio communication system according to thepreferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

In the preferred embodiment of the present invention, a move receivingwindow (MRW) transmission set indicator for SDU discard information ispreferably configured when a RLC layer is produced by an upper layer toestablish a radio bearer.

If the MRW mode set indicator is configured, for example, if a sequencenumber mode of a PDCP layer is set, the PDCP layers of transmission andreceiving sides reconcile the sequence numbers of PDCP PDUs, i.e. RLCSDUs. Therefore, the MRW mode set indicator should be configured. Thus,if the configured MRW mode set indicator is established, the RLC layerof the transmitting side transmits all the discard information regardingthe discarded SDUs to the corresponding RLC layer of the other radiocommunication system on the receiving side.

In this case, the discard information includes a sequence number of thePDU belonging to each end of the discarded SDUs. In detail, the discardinformation includes a field indicating the sequence number of the RLCPDU at an initiating position of the SDU next to each of the discardedSDUs. If two SDUs are overlapped with one PDU and the former SDU isdiscarded, the sequence number of the PDU indicates an end of thediscarded SDU. If a border between two SDUs coincides with that of twoPDUs and the former SDU is discarded, the sequence number of the secondPDU indicates an end of the discarded SDU.

Meanwhile, if the MRW mode indicator is not configured when the RLClayer is produced, for instance, if a sequence number mode of the PDCPlayer is not set, then the transmission side radio communication systemtransmits the discard information regarding the discarded SDUs to thereceiving side either entirely or in part. Of course, it is able totransmit all of the discard information to the receiving side.

In this case, a portion of the discard information, which is requiredfor moving a receiving side receiving window, indicates locationinformation of the last discarded SDU. Specifically, the portion of thediscard information indicates a sequence number of the PDU belonging toan end of the last discarded SDU. The discarded information about thediscarded SDUs, as mentioned in the above explanation, may be includedin the MRW SUFI.

A preferred construction of the MRW SUFI is shown in FIG. 3. Using thisconstruction, the entire discard information of the discarded SDUs canbe transmitted to the receiving side.

Referring to FIG. 3, the MRW SUFI preferably includes all of theparameters representing sequence numbers of all PDUs belonging to endsof the discarded SDUs. On the other hand, the MRW SUFI may beconstructed in accordance with the table shown in FIG. 4. Using thisalternative MRW SUFI, only a portion of the discard information of thediscarded SDUs is transmitted to the receiving side. Referring to FIG.4, the MRW SUFI according to the second embodiment includes oneparameter representing a sequence number of the PDU belonging to thelast discarded one of the discarded SDUs. Meanwhile, a portion of thediscard information of the discarded SDUs may be represented using theentire parameters like FIG. 3.

Referring to FIG. 3, if the MRW mode set indicator is configured for theRLC layer, as mentioned in the above description, the discardinformation of the entire SDUs discarded in the transmitting side shouldbe transmitted to the receiving side. Therefore, the MRW SUFI preferablyincludes the parameters LENGTH, SN_MRW1, SN_MRW2, . . . ,SN_MRWLENGTH−1, SN_MRWLENGTH, and NLENGTH.

The parameter LENGTH preferably comprises 4 bits, and indicates thenumber of the discarded SDUs. The parameters SN_MRW1, SN_MRW2, . . . ,SN_MRWLENGTH−1, and SN_MRWLENGTH represent sequence numbers of PDUsbelonging to ends of the discarded SDUs. Each parameter SN_MRWipreferably comprises 12 bits.

The parameter NLENGTH indicates which SDUs so far correspondsequentially to the last discarded SDU having the PDU belonging to (orrepresenting) an end of the last discarded SDU.

If the number of the discarded SDUs is four, the parameter LENGTH isrepresented by 4 bits of “0100”. Meanwhile, each SN_MRWi (i=1, 2, . . ., LENGTH−1, LENGTH) indicates a sequence number of each PDU belonging toan end of each of the discarded SDUs. For example, if the SDU next tothe second discarded SDU starts from the second PDU having a sequencenumber of “2”, the parameter SN_MRW2 is represented by the sequencenumber of “2”. In this case, the parameter for the location informationof the discarded SDU is represented by the sequence number of the PDUfrom which the next SDU starts.

On the other hand, if the MRW mode set indicator is not configured forthe RLC layer, as mentioned in the above description, the radiocommunication system corresponding to the transmitting side just informsthe receiving side of the discard information required for moving awindow of the receiving side. In this case, the radio communicationsystem corresponding to the transmitting side loads the locationinformation of the last one of the discarded SDUs on the MRW SUFI.

Therefore, FIG. 4 includes the parameter SN_MRWLENGTH, which representsthe sequence number of the PDU which belongs to (indicates) an end ofthe last discarded SDU. FIG. 4 further shows another parameter NLENGTH,which indicates that the last discarded SDU corresponds sequentially tothe SDU in the indicated PDU. The parameter NLENGTH represents thenumber of SDU(s) to be ignored in the indicated PDU.

In FIG. 4, the parameter LENGTH of the MRW SUFI represents thetransmitted information of the last discarded SDU. Further, thetransmitting side radio communication system checks whether the lastdiscarded SDU exceeds a capacity of the transmission window. In thiscase, the parameter LENGTH, which is different from that shown in FIG.3, indicates the check result. Such a field, however, may not be used.

When the SDU, which is discarded last and for which discard informationhas been transmitted, is located in the transmission window, the radiocommunication system sets the parameter LENGTH as “0001”. When the lasttransmitted SDU exceeds or is out of the range of the transmissionwindow, however, the field of the parameter LENGTH is set as “0000”.

As mentioned in the foregoing description, when the discard informationof the transmitting side is transmitted to the receiving side, thetransmitting side radio system transmits the information of thediscarded SDUs, either entirely or in part, to the receiving sideaccording to whether the transmission MRW mode set indicator isconfigured for its RLC layer by the RRC layer.

As described above, the Move Receiving Window super-field is used torequest the RLC receiver to move its receiving window and optionally toindicate the set of discarded SDUs, as a result of an SDU discard in theRLC transmitter. The format is shown in FIGS. 3 and 4.

Thus, referring to FIGS. 3 and 4, the Length field is preferably 4 bits.It is used to indicate the number of SN_MRWi fields in the super-fieldof type MRW. The values “0001” through “1111” indicate 1 through 15SN_MRWi respectively. The value “0000” indicates that one SN_MRWi fieldis present and that the SDU to be discarded in the receiver extendsabove the configured Tx window in the transmitter.

The fields SN_MRWi are preferably 12 bits. When a transmission mode MRWis configured, such as a Send MRW, a SN_MRWi is used to indicate the endof each discarded SDU. That is, the number of SN_MRWi fields equals thenumber of SDUs discarded by that MRW SUFI. When the transmission modeMRW is not configured, however, SN_MRWi is used to indicate the end ofthe last SDU to be discarded in the receiver. It may also optionally beused to indicate the end of other discarded SDUs. Preferably, SN_MRWi isthe sequence number of the PDU that contains the length indicator (LI)of the ith SDU to be discarded in the receiver. However, this is not thecase for SN_MRWLENGTH when NLENGTH=0. The order of the SN_MRWi ispreferably in the same sequential order as the SDUs to which they refer.

Additionally SN_MRWLENGTH requests the RLC receiver to discard all PDUswith sequence numbers that are less than SN_MRWLENGTH, and to move thereceiving window accordingly. In addition, when NLENGTH is greater than0, the receiver preferably discards the first NLENGTH LIs and thecorresponding data octets in the PDU with sequence number SN_MRWLENGTH.

The NLENGTH field is preferably 4 bits, and is preferably used togetherwith SN_MRWLENGTH to indicate the end of the last SDU to be discarded inthe receiver. NLENGTH indicates which LI in the PDU with sequence numberSN_MRWLENGTH corresponds to the last SDU to be discarded in thereceiver. A NLENGTH equal to 0 indicates that the last SDU ended in thePDU with sequence number SN_MRWLENGTH−1 and that the first data octet inthe PDU with sequence number SN_MRWLENGTH is the first data octet to bereassembled next.

FIG. 5 illustrates a process of carrying out the discard function usinga message structure of the MRW SUFI shown in FIG. 3. Referring to FIG.5, parameters included in the MRW SUFI are explained when the RLC SDUsare discarded.

It is assumed that the RLC SDUs coming down to the RLC layer of thetransmitting side are being transmitted to the receiving side. In thiscase, SDU10 to SDU13 may be discarded if the previously set transmissiontime in the timer for SDU10 to SDU13 expires or if the prescribed numberof transmission is limited.

For instance, if SDU10 to SDU13 are transmitted to the receiving sidebut are lost during the transmission, or if the transmitting side failsto receive a positive acknowledgment from the receiving side, thetransmitting side discards SDU10 to SDU13. The discarded informationthat is loaded on the MRW SUFI is then transmitted to the receivingside.

The MRW SUFI includes the location information of each of the discardedSDUs. Thus, the MRW SUFI in FIG. 3 includes the location information ofthe four RLC SDUs corresponding to SDU10 to SDU13. Hence, a value of theparameter LENGTH, as shown in FIG. 5, becomes “4”. Additionally, valuesof the four parameters SN_MRW1 to SN_MRW4 representing the sequencenumbers of the PDUs that belong to or indicate ends of the respectivediscarded SDUs are “1”, “1”, “3”, and “7”, respectively.

The last parameter SN_MRW4 is set to “7,” which corresponds to PDU7,which is the end of the discarded SDU13. In addition, the last parameterSN_MRW4 directs a start of the following SDU14, and means that PDU7 hasa transmission priority when the transmission is reinitiated. SDU13 isthe only discarded SDU in PDU7. Hence, a value of the parameter NLENGTHis “1.” This commands that the first SDU of the PDU7 transmitted fromthe receiving side is to be ignored and that the second SDU should bereceived.

As shown in FIG. 5, the receiving side, having received the MRW SUFI,transmits a response signal MRW_ACK SUFI. The MRW-ACK SUFI correspondsto the MRW SUFI, and is sent to the transmitting side. The receivingside also discards the corresponding RLC SDUs according to the contentsincluded in the MRW SUFI. The receiving side then moves the receivingwindow to the location of the last discarded SDU. In FIG. 5, thereceiving side moves the receiving window to the PDU having the sequencenumber “7”. The receiving side also transfers the discard information ofthe RLC SDUs discarded by the transmitting side to an upper layer.

FIG. 6 illustrates a process of carrying out the discard function usingthe MRW SUFI shown in FIG. 4. The process of FIG. 6 is frequently usedwhen the transmission MRW mode indicator is not configured by the RRC.In FIG. 6, SDU10 to SDU13 are discarded due to either the expiration ofthe previously-set time of the timer or the limitation of thetransmission number.

As previously mentioned, when the transmission MRW mode indicator is notconfigured, it is unnecessary to transmit the entire discard informationfor all of the discarded SDUs to the receiving side. As explained inFIG. 4, it is only necessary to inform the receiving side of thelocation information of the last discarded one of the discarded SDUs.

Therefore, the MRW SUFI includes the location information of the lastdiscarded SDU, in this case SDU13, which fails to exceed thetransmission window. Thus, a value of the parameter LENGTH in FIG. 6becomes “1”. Moreover, as shown in FIG. 6, the RLC PDU belonging to orindicating the end of the last discarded SDU13 is PDU7, and SDU 13 isthe first one discarded from PDU7. Therefore, a value of the parameterSN_MRW1 is “7”, and a value of the parameter NLENGTH becomes “1”.

The receiving side, having received the MRW SUFI, discards PDUs PDU0 toPDU6. The discarded PDUs are those which have sequence numbers equal toor lower than “7,” which is the parameter SN_MRW included in the MRWSUFI. If receiving PDU7, the receiving side discards PDU7 as well. Thereceiving side then moves the receiving window to PDU7 having thesequence number of “7”.

Because the receiving side does not have the transmission MRW modeindicator configured, it does not send the discard information regardingthe discarded SDU10 to SDU13 up to the upper layer. In this case, thesequence numbers, if existing, of PDCP PDUs (corresponding to the RLCSDUs) fail to coincide with each other.

An operation for performing the discard function of radio communicationsystems corresponding to the transmitting and receiving sides isexplained in detail as follows by referring to FIG. 7 and FIG. 8.

FIG. 7 is a flowchart illustrating an operation of the discard functionsof a radio communication system corresponding to transmitting andreceiving sides respectively. FIG. 8 illustrates a process of a discardfunction for service data units according to a transmission time set ina timer and a limited transmission number in a radio communicationsystem according to a preferred embodiment of the present invention.

Referring to FIGS. 7 and 8 the time-based service data units RLC SDU0 toRLC SDU4 are successively transmitted as PDU forms, such as PDU0 toPDU7, from a transmitting side of the radio communication system to thereceiving side (Step 710).

The respective protocol data units PDU0 to PDU7 corresponding to therespective service data units SDU0 to SDU4 are thus transmitted alongwith their sequence numbers SN=0 to SN=7 to the receiving side. Duringtransmission, the transmitting side determines whether thepreviously-set transmission time of the RLC service data units RLC SDU0to RLC SDU3 has expired or whether the previously-set transmissionnumber has exceeded the limit (Step 720).

If the previously-set transmission time of the RLC service data unitsRLC SDU0 to RLC SDU4 fails to expire, or if the previously-settransmission number fails to exceed the limit, the transmitting siderepeatedly transmits RLC SDU0 to RLC SDU4 to the receiving side. In thiscase, the transmitting side checks whether the respective RLC SDU0 toRLC SDU4 have been transmitted to the receiving side by confirming thereception of the transmitted PDU.

If the confirmation is not made before the previously-set transmissiontime of the RLC service data units RLC SDU0 to RLC SDU4 expires or thepreviously-set transmission number exceeds the limit, the transmittingside determines that transmission of the respective RLC SDU0 to RLC SDU4has failed.

If RLC SDU0 to SDU4 are transmitted to the receiving side, thetransmitting side checks whether the transmission MRW mode indicator isconfigured for/to the RLC layer (Step 740). If it is determined that therespective RLC SDU0 to RLC SDU4 have not been transmitted to thereceiving side, the transmitting side discards the corresponding RLCSDUs (Step 730). If the transmission MRW mode indicator is configured,the transmitting side discards the transmitted RLC SDUs, transmits theentire discard information about the discarded SDUs to the receivingside, and transmits PDU information corresponding to the SDU to betransmitted thereafter.

If, on the other hand, the transmission MRW mode indicator is notconfigured, the transmitting side transmits a portion of the discardinformation to the receiving side. In this case, it is a matter ofcourse that the entire discard information for the respective discardedSDUs may be transmitted to the receiving side. The previous discussionof the discard information and the MRW SUFI on which the discardinformation is loaded is incorporated herein by reference.

If the previously-set transmission time of the respective RLC servicedata units RLC SDU0 to RLC SDU4 expires or if the previously-settransmission number exceeds the limit, and if at least portions of RLCSDU0 to RLC SDU4 fail to be transmitted to the receiving side, thetransmitting side sequentially numbers the sequence numbers of the PDUscorresponding to the SDUs to be transmitted subsequently thereafter fromthe sequence number of the presently-not-transmitted PDU.

Then, it is determined whether the transmission MRW mode indicator isconfigured or not. This process is described above, and a detailedexplanation is thus omitted.

Meanwhile, if the receiving side requests transmission when the discardinformation has been entirely or partially transmitted to the receivingside, the transmitting side sequentially transmits the numbered RLC PDUsto the receiving side.

Next, a process of renumbering the sequence numbers of the PDUs isdescribed in accordance with whether an attempt has been made totransmit the respective RLC SDUs.

Referring to FIG. 8, an example of applying the time-based discardmethod is shown.

First, the RLC layer of the transmitting side of the radio communicationsystem transmits RLC SDU0 to RLC SDU4, which are received from an upperlayer, to the receiving side.

In this case, the transmitting side transmits PDUs corresponding to RLCSDU0 to RLC SDU4 to the receiving side. As shown in FIG. 8, RLC SDU0 istransmitted to the receiving side through RLC PDU0 and RLC PDU1. RLCSDU1 is transmitted to the receiving side through RLC PDU1, RLC SDU2 istransmitted to the receiving side through RLC PDU1, RLC PDU2, and RLCPDU3, and RLC SDU3 is transmitted to the receiving side through RLC PDU3to RLC PDU6. The RLC PDUs have sequence numbers SN=0, SN=1, SN=3, SN=4,SN=5, and SN=6 in order.

Referring to FIG. 8, the transmitting side transmits RLC PDU0 to RLCPDU2, which correspond to RLC SDU0 to RLC SDU2 respectively. RLC PDU0and RLC PDU1 are received by the receiving side, which transmitsacknowledgment signals ACK. In this case, however, the acknowledgmentsignal is lost and not received by the transmitting side. In fact, thetransmitting side is unable to know whether the receiving side failed toreceive or whether the transmitting side failed to receive the receivingconfirmation signal.

Additionally, RLC PDU2 is lost during the transmission from thetransmitting side to the receiving side, and RLC PDU3 to RLC PDU6 arenot yet transmitted. In this case, the transmitting side is unable toknow that the receiving side failed to receive RLC PDU2, which was lostduring the transmission, or that the transmitting side itself failed toreceive the receiving confirmation signal. Also, because RLC PDU3 wasnever transmitted, the transmitting side can confirm that the receivingside failed to receive it.

In this case, if the previously set buffer stay transmission time of RLCSDU0 to RLC SDU3 exceeds the prescribed time limit, the transmittingside of the radio communication system discards the transmitted PDUs(RLC PDU0 to RLC PDU2) and RLC SDU3 corresponding to the RLC PDU3 to RLCPDU6. It also discards RLC SDU0 to RLC SDU2 corresponding to the RLCPDU0 to RLC PDU2, which are stored in the transmission buffer and forwhich transmission has been attempted.

Meanwhile, the transmission side of the radio communication systemchecks which SDUs of the discarded RLC SDUs have had a transmissionattempt, and determines a sequence number of the first RLC PDU to whichdata of the discarded RLC SDU belongs. In FIG. 8, the sequence number SNof the first RLC PDU to which data of the discarded RLC SDU belongs andwhich has never been transmitted is “3”. Moreover, the transmittingside, as explained above, transmits the discard information of thediscarded RLC SDUs to the receiving side using the MRW SUFI. In thiscase, the discard information includes PDU information relating to theSDU to be transmitted next time.

The transmitting side renumbers the sequence numbers of PDU(s)corresponding to RLC SDU4, which is to be transmitted later, from thesequence number SN=3 of the never-transmitted and discarded PDU3.Additionally, an initial point of the SDU becomes equal to that of thePDU.

Therefore, as shown in FIG. 8, the sequence number SN of PDU7, whichcorresponds to RLC SDU4 and is to be transmitted after the RLC SDU3, ischanged from “7” to “3.” The subsequent process of confirming that thetransmission MRW SUFI is configured is the same as that shown in FIG. 7.

Meanwhile, the receiving side of the radio communication system receivesthe discard information, and discards RLC PDU0 to RLC PDU6 correspondingto RLC SDU0 to RLC SDU3.

Subsequently, the receiving side transmits a demand signal to thetransmitting side. The demand signal commands that the RLC PDU having asequence number of “3” be transmitted from the transmitting side in thefirst place.

The transmitting side transmits the next RLC SDU4 with the RLC PDU7having the sequence number SN=3 to the receiving side. Also, sequencenumbers of RLC PDUs after the RLC PDU7 are renumbered consecutively froma number right after the sequence number SN=3. Moreover, the RLC PDUshaving the renumbered sequence numbers are transmitted from thetransmitting side to the receiving side in response to the demandsignal.

A flowchart for the above processing is shown in FIG. 9.

The discard method of the preferred embodiment has many advantages. Forexample, the RLC layer of the transmitting side causes PDUs, whichcorrespond to SDUs which are to be transmitted, to be transmitted to thereceiving side. In this case, the success/failure of reception isconstantly monitored. The time limit is imposed on SDUs staying in thetransmission buffer without confirming the success/failure of reception.If this time limit expires, the corresponding SDUs are discarded and theretransmission is terminated. Additionally, information regarding thediscarded SDU(s) is transmitted to the receiving side. It is alsodetermined whether a PDU having data of a specific SDU is evertransmitted to the receiving side. Hence, a sequence number of the PDUcorresponding to the SDU for which transmission has never been attemptedis used for a sequence number of another PDU corresponding to new SDU.

Moreover, the receiving side has no information about the SDU, wherebythe discard information is not transmitted either. Yet, according to thepreferred embodiment, it is unable to do so when sequence numbers areused in the PDCP layer. Therefore, the preferred embodiment isapplicable when the transmission MRW mode indicator is not configured.

In the transmitting side of the radio communication system according tothe preferred embodiment, the discard information about the RLC SDUsdiscarded in the RLC layer is transmitted entirely or partially to theRLC layer of the receiving side variably in accordance with theconfiguration of the transmission MRW mode indicator. Therefore, the SDUdiscard function of the preferred embodiment can be carried out withonly a control signal. Thus, less information is processed and the loadof the transmitting side can be reduced.

Moreover, according to the preferred embodiment, the sequence numbers ofthe RLC PDUs are renumbered in accordance with success/failure of thetransmission of the RLC PDUs corresponding to the very RLC SDUs. Whenthe transmitting side sends the discard information to the receivingside, bit amounts used for informing the discard information about thediscarded service data units are used in part. Specifically, only therequired discard information is transmitted to the receiving side,thereby reducing the amount of signals required for the transmission.Therefore, a limited radio resource is used efficiently.

The foregoing embodiments and advantages are merely exemplary and arenot to be construed as limiting the present invention. The presentteaching can be readily applied to other types of apparatuses. Thedescription of the present invention is intended to be illustrative, andnot to limit the scope of the claims. Many alternatives, modifications,and variations will be apparent to those skilled in the art. In theclaims, means-plus-function clauses are intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents but also equivalent structures.

What is claimed is:
 1. A method of transmitting data from a radio linkcontrol (RLC) layer operating in an acknowledged mode in a radiocommunication system, the method comprising: receiving service dataunits (SDUs) from an upper layer; transforming the service data unitsinto protocol data units (PDUs) to be transmitted to a receiving side;transmitting at least one protocol data unit (PDU) among the protocoldata units (PDUs) to the receiving side; and discarding at least oneservice data unit (SDU) if a discard time limit expires for the at leastone service data unit (SDU), determining whether a transmission modeindicator is configured; and transmitting one of complete discardinformation and partial discard information relating to the discarded atleast one SDU that has been transmitted to the receiving side inaccordance with whether the transmission mode indicator is configured.2. The method of claim 1, wherein the discard information about thediscarded service data unit (SDU) corresponds to a moving positioninformation of a receiving window of the receiving side.
 3. The methodof claim 1, wherein the discard information about the discarded servicedata unit (SDU) is loaded to a receiving window moving super field MRWSUFI to be transmitted to the receiving side.
 4. The method of claim 3,wherein the receiving window moving super field comprises a LENGTH fieldindicating whether or not a last transmitted service data unit (SDU) ofthe discarded at least one service data unit (SDU) exceeds a range of atransmission window, a SN_MRW_(LEGNTH) field indicating a sequencenumber of a last protocol data unit (PDU) including the last transmittedservice data unit (SDU), and a N_(LENGTH) field indicating a specificorder of the last transmitted service data unit (SDU) in the lastprotocol data unit (PDU) having the sequence number.
 5. The method ofclaim 4, wherein the LENGTH field is set as “0000” when the lasttransmitted service data unit (SDU) of the discarded at least oneservice data unit (SDU) exceeds a range of the transmission window. 6.The method of claim 1, wherein the discard information about thediscarded service data unit (SDU) indicates the end of the lasttransmitted service data unit (SDU) among the discarded at least oneservice data unit (SDU).
 7. The method of claim 6, wherein the discardinformation is a sequence number of a protocol data unit (PDU) to whichthe last transmitted service data unit (SDU) belongs.
 8. The method ofclaim 1, wherein the discard information further comprises informationabout a protocol data unit (PDU) which includes a service data unit(SDU) to be transmitted next to the receiving side.
 9. A transmitter fortransmitting data to a receiving side in a radio communication system,the transmitter configured to: receive service data units (SDUs) from anupper layer; transform the service data units into protocol data units(PDUs) to be transmitted to a receiving side; transmit at least oneprotocol data unit (SDU) among the protocol data units (PDUs) to thereceiving side; and discard at least one service data unit (SDU) if adiscard time limit expires for the at least one service data unit (SDU),determine whether a transmission mode indicator is configured; andtransmit one of complete discard information and partial discardinformation relating to the discarded at least one SDU that has beentransmitted to the receiving side in accordance with whether thetransmission mode indicator is configured.
 10. The transmitter of claim9, wherein the discard information about the discarded service data unit(SDU) corresponds to a moving position information of a receiving windowof the receiving side.
 11. The transmitter of claim 9, wherein thediscard information about the discarded service data unit (SDU) isloaded to a receiving window moving super field MRW SUFI to betransmitted to the receiving side.
 12. The transmitter of claim 9,wherein the discard information about the discarded service data unit(SDU) indicates the end of the last transmitted service data unit (SDU)among the discarded at least one service data unit (SDU).
 13. Thetransmitter of claim 12, wherein the discard information is a sequencenumber of a protocol data unit (PDU) to which the last transmittedservice data unit (SDU) belongs.
 14. The transmitter of claim 9, whereinthe discard information further comprises information about a protocoldata unit (PDU) which includes a service data unit (SDU) to betransmitted next to the receiving side.
 15. The transmitter of claim 11,wherein the receiving window moving super field comprises a LENGTH fieldindicating whether or not a last transmitted service data unit (SDU) ofthe discarded at least one service data unit (SDU) exceeds a range of atransmission window, a SN_MRW_(LEGNTH) field indicating a sequencenumber of a last protocol data unit (PDU) including the last transmittedservice data unit (SDU), and a N_(LENGTH) field indicating a specificorder of the last transmitted service data unit (SDU) in the lastprotocol data unit (PDU) having the sequence number.
 16. The transmitterof claim 15, wherein the LENGTH field is set as “0000” when the lasttransmitted service data unit (SDU) of the discarded at least oneservice data unit (SDU) exceeds a range of the transmission window.